1. Technical Field
The present invention relates to an optical element and an optical apparatus.
2. Related Art
H. Tamada, et al., “Al wire-grid polarizer using the s-polarizationresonance”, Opt. Lett. 22, 6, pp. 419-421 (1997) and JP-T-2003-508813 describe technologies of a polarizing element (polarizing filter) formed of a wire-gird element having a metal wire structure on a light-transmissive substrate.
JP-T-2010-530995 describes a technology of a wire-grid element that has a wire structure formed of a metal portion, a transparent dielectric portion, a light absorbing portion, and another transparent dielectric portion on a light-transmissive substrate and selectively suppresses reflection of polarized light.
JP-A-2011-123474 describes a technology of a polarizing element having an additional function of selectively absorbing unnecessary polarized light. The additional function is achieved by forming a wire-grid element having a metal wire structure on a light-transmissive substrate and shaping the upper end of the metal wire structure to have periodic protrusions.
JP-A-2009-104074 describes a technology of an optical element that has metal portions disposed in a glass layer in what is called a checker flag pattern so that one type of polarized light is converted into another type of polarized light.
Optical apparatus are widely used for ordinary use. For example, a liquid crystal projector, a display, an optical pickup, and an optical sensor use many optical elements that control light. As these apparatus advance in terms of functionality, an optical element is also required to have more advanced functionality, higher added values, and lower cost.
A representative example of such an optical apparatus is a liquid crystal projector. A liquid crystal projector includes a first polarizing element that receives a light flux emitted from a light source and selectively transmits specific polarized light out of the received light flux, a liquid crystal panel that receives the polarized light from the first polarizing element and changes the polarization direction of the incident polarized light in accordance with image information, and a second polarizing element that receives the polarized light having the polarization direction changed by the liquid crystal panel. In the liquid crystal projector, the polarized light having emitted out of the second polarizing element described above is incident on a projection lens, which forms an optical image (image light) and projects the image light on a screen or any other surface for image display. In the thus configured liquid crystal projector, the first polarizing element and the second polarizing element are so disposed that they sandwich the liquid crystal panel. That is, a polarizing element (polarizing filter) having a function of selectively transmitting specific polarized light is disposed both on the light incident side and the light exiting side of the liquid crystal panel.
In recent years, the optical density on the liquid crystal panel is increased to achieve a reduced size of a liquid crystal projector and increased brightness of a projected image, and a polarizing element is desired to have excellent heat and light resistance in correspondence with the increased optical density. In this regard, for example, it can be said that a wire-grid element made of an inorganic material is suitable for such a polarizing element.
H. Tamada, et al., “Al wire-grid polarizer using the s-polarization resonance”, Opt. Lett. 22, 6, pp. 419-421 (1997) describes a definition of a wire-grid element as follows: “A wire grid is a simple one-dimensional metal grating and is quite promising as a microminiaturized polarization component in the field of integrated optics.” That is, a wire-grid element is a simple one-dimensional metal grating.
For example, from a viewpoint of improving image quality provided by an optical apparatus (image projection apparatus) a representative example of which is a liquid crystal projector, it is desired to improve performance of selecting specific polarization (hereinafter referred to as p-polarization) and polarization perpendicular thereto (hereinafter referred to as s-polarization), that is, a transmittance ratio, which is one of primary performance indices of a polarizing element. In the present specification, a polarization contrast ratio (Tp/Ts), where Tp represents p-polarization transmittance and Ts represents s-polarization transmittance, (also called extinction ratio) is used as a performance index of the polarization selectivity. In this case, the higher the polarization contrast ratio, the more excellent polarization selectivity a polarizing element has.
As described above, a wire-grid element has linear metal wires arranged in equal periodicity on a light-transmissive substrate. In the thus configured wire-grid element, the performance thereof depends on the period and shape of the metal wires, and constraints are imposed thereon. For example, it is known that the period between the metal wires has an upper limit determined by a Rayleigh resonance phenomenon. As a specific example, when the wavelength to be used is 450 nm, the period between the metal wires needs to be smaller than or equal to about 210 nm, as described in the paragraph in JP-T-2003-508813. Further, in a wire-grid element, Tp and the polarization contrast ratio, which are performance indices, are related to the width and height of the metal wires in a tradeoff relationship. In view of the fact described above, a wire-grid element is typically so designed that the width of the metal wires is set to be about one-third the period (pitch) therebetween and the height of the metal wires is set at about 150 nm.
The constraints described above on the shape of the metal wires and the period between the arranged metal wires make it difficult to greatly change design conditions under which a wire-grid element is designed. Therefore, for example, even when it is attempted to improve the image quality of an optical apparatus a representative example of which is a liquid crystal projector by improving the polarization contrast ratio of a polarizing element, it is difficult to improve the current performance of the polarizing element or the optical apparatus unless the metal wire structure of a wire-grid element is changed.